Processes for producing polyethylene are roughly divided into a high-pressure radical polymerization using a radical forming agent process and a high-, medium- or low-pressure ionic polymerization process using a Ziegler catalyst.
A high-pressure radical polymerization process generally produces low-density polyethylene having a density of from 0.91 to 0.935 g/cm.sup.3, and a high-, medium- or low-pressure ionic polymerization process gives high-, medium- or low-density polyethylene having a density of from about 0.91 to 0.97 g/cm.sup.3 by introducing an .alpha.-olefin as a copolymerizable monomer.
On comparing the structure and physical properties between the low-density polyethylene from high-pressure radical polymerization (hereinafter referred to as high-pressure-processed LDPE) and the low-density polyethylene from high-, medium- or low-pressure ionic polymerization (hereinafter referred to as LLDPE), there is an essential difference in molecular structure, though the densities are on the same level. That is, LLDPE has short-chain branches introduced by copolymerizing an .alpha.-olefin, applying an extended technique of the production of linear high-density polyethylene, to thereby reduce the density, whereas high-pressure-processed LDPE has its density reduced by long-chain branches produced by radical polymerization.
High-pressure-processed LDPE and LLDPE differ in physical properties resulting from the above-described structural difference and accordingly differ in application. In more detail, since high-pressure-processed LDPE has a relatively broad molecular weight distribution and long-chain branches, it is excellent in optical characteristics, such as gloss and transparency, and is therefore suitable for extrusion molding to produce films, etc. On the other hand, since LLDPE obtained by ionic polymerization has short-chain branches and a narrow molecular weight distribution, it becomes highly viscous when molded at a high shear rate by, for example, extrusion molding and is also inferior in optical characteristics to high-pressure-processed LDPE, but is suitable for injection molding and provides molded articles having a high melting point and excellent mechanical strength.
There has recently been a demand for high-pressure-processed LDPE resins with characteristics not possessed by the conventional polyethylene resins, for example, improved rigidity, printability, lubricity, and adhesiveness, in addition to the characteristics inherent to polyethylene.
Further, in the field of structural materials and automobile parts, etc., it has been demanded to endow polyamide, polyester, polyphenylene ether, ABS resins, polycarbonate, polyphenylene sulfide, and the like, which are typical engineering plastics, with improved resistance to impact, oils, and chemicals. To meet this demand, compounding of these engineering plastics with a polyethylene resin has been proposed in, for example, U.S. Pat. Nos. 4,397,982 and 4,554,316, but, as hereinafter described, none of conventional high-pressure-processed LDPE type polyethylene resins is miscible with the engineering plastics with good compatibility.
Typical techniques for modifying polyethylene include a process comprising graft polymerizing a vinyl monomer, e.g., acrylic acid, methacrylic acid (hereafter collectively referred to as "(meth)acrylic acid") or a derivative thereof and maleic anhydride, to polyethylene as disclosed, for example, in JP-B-39-6384 and JP-B-42-10727 (the term "JP-B" as used herein means an "examined and published Japanese patent application").
However, according to the above-described grafting process in which a graft monomer is radical polymerized in the presence of an ethylene homopolymer using an organic peroxide or radiation as a polymerization initiator (hereinafter called a general-purpose grafting process), homopolymerization of the graft monomer preferentially takes place only to produce a graft copolymer with short-chain branches at a low purity, having caused difficulty in greatly improving physical properties of polyethylene.
Besides the above-described modification process, high-pressure radical polymerization of an ethylene monomer and a polyalkylene glycol monoacrylate having a degree of condensation as low as 4 to 9 has been proposed as a process for producing modified high-pressure-processed LDPE as disclosed in JP-B-61-28685.
This process provides an ethylene random copolymer excellent in hydrophilic and antistatic properties. The random copolymer, however, carries short-chains derived from the polyalkylene glycol unit and is, therefore, limited in their use similarly to the graft copolymers obtained by the general-purpose grafting process. For example, it is barely satisfactory as a resin to be compounded into the engineering plastics or as a compatibilizer to be used in mixing or compounding polyethylene and the engineering plastics.
In the case of LLDPE, on the other hand, U.S. Pat. No. 3,786,116 suggests, as a means for modifying LLDPE, an ethylene graft copolymer obtained by polymerizing ethylene and a high-molecular-weight monomer having a molecular weight of from 5,000 to 50,000, called a macromonomer, which comprises polystyrene, poly-.alpha.-methylstyrene, etc. as a polymer skeleton with an .alpha.-olefinic double bond at the terminal thereof, in the presence of a Ziegler catalyst. The ethylene graft copolymer exhibits marked effects of modifying polyethylene even with a small proportion of the macromonomer and is useful as a compatibilizer, and the like.
Hence, it has been studied to apply the above-described copolymerization of a macromonomer and ethylene to the production of an ethylene graft copolymer comprising high-pressure-processed LDPE as a main chain and a macromonomer as a grafted component. However, it is generally received that radical polymerization of a macromonomer under such a high temperature and high pressure condition as in the synthesis of high-pressure-processed LDPE encounters extreme technical difficulty for the reasons set forth below. There is found no report in which a macromonomer and ethylene are random copolymerized under a high temperature and high pressure condition for the purpose of modifying high-pressure-processed LDPE.
When exposed to the above-described reaction condition, macromonomers, particularly those having polymethyl methacrylate or polystyrene as a polymer skeleton tend to undergo depolymerization or molecular cut-off. Should it occur, radical concentration in the polymerization system would abnormally increase, resulting in the failure of control of polymerization. Moreover, it appears that the macromonomer acts as a chain transfer agent so that a desired high-molecular polymer cannot be obtained.